Class Act: Cardiac CT to Assess Coronary Artery Calcium

The patient: a 55 year old male nonsmoker with an HDL of 46, LDL of 120, triglycerides of 70, BP of 135/80 (on meds) and total cholesterol of 180. (Framingham 10-year risk of MI = 12%)

Coronary artery disease is the leading cause of death worldwide. Despite our understanding of how family history, toxic habits, cholesterol, and blood pressure affect the risk of myocardial infarction (MI), there is a clear need to further refine the methods of risk stratification. The Framingham Risk Score (FRS) is an excellent in-office tool that can be used to determine whether a patient is at low (<10%), intermediate (10%-20%), or high (>20%) 10-year risk for future coronary heart disease (CHD) events based on history and basic lab tests. Patients determined to be at low risk by this method rarely suffer cardiac events. On the other hand, high-risk patients are advised to follow the most aggressive pharmacologic and preventive measures available; this approach has greatly reduced the number of adverse outcomes in this population. It is, however, patients identified as intermediate risk by the FRS who now account for the majority of CHD events.

Clearly there is a need to further identify risk factors and screening modalities that can indicate which intermediate-risk patients are at greater risk and might benefit from more aggressive preventive measures. One such possibility is the use of cardiac CT to assess coronary artery calcium (CAC).

Coronary artery calcifications are absent in normal vessels, but are highly correlated with coronary artery disease (CAD). They are therefore pathognomonic for atherosclerosis and can now be reliably detected by CT scanning. When these scans are evaluated, the patient is assigned a score that represents the coronary artery calcification burden. Cardiac CT to identify CAC should not be confused with coronary CT angiography, which is a different modality involving the use of IV contrast and additional radiation. Although the presence of calcification does not necessarily correlate with the degree of stenosis, total atherosclerotic burden has actually been shown to be an excellent determinant of future MI risk. A meta-analysis seeking to discover how well coronary artery calcium scoring predicts clinical events found the risk of major coronary events increased 2.1-fold for scores ranging from 1-100 and 10-fold for scores >400, compared with scores of 0. To evaluate the implications of a negative result, two large studies demonstrated a posttest probability of coronary events to be 0.1% per year if no CAC is detected in asymptomatic individuals.

Despite its apparent power, cardiac CT does have potential risks and limitations. There is a not insignificant financial cost (although it remains relatively cheap) and it obviously involves radiation, although recent technologic improvements have substantially reduced the level of exposure.9 Furthermore, although cardiac CT is highly sensitive for detecting coronary artery calcifications, the specificity (detecting lesions that will actually cause MI) is impossible to gauge and there will surely be some degree of overdiagnosis. The risk to these patients involves the anxiety of a positive test result and the increased possibility of side effects associated with more aggressive medical treatment of cholesterol and blood pressure. Cardiac CT, by nature, also images the surrounding structures, which can lead to incidental findings. In one study, lung nodules were noted in 4.9% of patients. The vast majority of these nodules turned out to be pathologically benign, but “incidentalomas” certainly can cause increased anxiety and lead to further unnecessary diagnostic tests.

So who should be offered cardiac CT? A 2007 AHA/ACCF “Expert Consensus Document on Coronary Artery Calcium Scores” evaluated the available literature and found that a CAC score greater than or equal to 400 in a patient with an intermediate FRS represents a 10-year coronary heart disease risk similar to that of diabetes or peripheral arterial disease and therefore represents a CAD equivalent. These patients could be re-classified as “high-risk” and aggressive prevention could be recommended. The expert consensus concluded that CAC testing was “reasonable” to further risk stratify and guide therapy of these patients. The European Society of Radiology and North American Society for Cardiovascular Imaging went a step further in a joint statement that recommended the use of cardiac CT for risk stratifying intermediate populations. Even more aggressive, the SHAPE task force recommended that all men between 45 and 75 years of age and all women between 55 and 75 years of age (except those identified as “very low risk”) be screened by cardiac CT or carotid intima media thickness and treated accordingly. However, most professional organizations have not yet been so clear in their support. In 2007, the US Preventive Services Task Force assigned calcium screening a level “I” recommendation (for insufficient data).

The mounting evidence suggests that knowledge of CAC can be greatly beneficial in directing the treatment protocols of patients with intermediate FRS’s. Currently, CAC is one in a growing number of possible diagnostic tests (e.g. carotid intima media thickness, C-reactive protein, and biomarker blood tests) that are being evaluated for efficacy in CHD risk stratification of intermediate-risk patients. Although the need is clear, which testing modality will ultimately be found the most beneficial remains to be seen.

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